Cathode for metal-air current sources metal-air current sources containing the same

ABSTRACT

The invention relates to electrochemical current sources, more particularly to metal-air current sources, and even more particularly to lithium-air current sources and their electrodes. A cathode comprises a base made of a porous electrically conducting material that is permeable to molecular oxygen, the working surface of which has a copolymer applied thereto, which is produced by the copolymerization of a monomeric transition metal coordination complex having a Schiff base and a thiophene group monomer. The monomeric transition metal coordination complex having a Schiff base can be, for example, a compound of the [M(R,R-Salen)], [M(R,R-Saltmen)] or [M(R,R-Salphen)] type, and the thiophene group monomer can be a compound selected from a thiophene group consisting of 3-alkylthiophenes, 3,4-dialkylthiophenes, 3,4-ethylenedioxythiophene or combinations thereof. A current source comprises the described cathode and an anode made from an active metal, in particular lithium, wherein the cathode and the anode are separated by an electrolyte containing ions of the metal from which the anode is made. It has been established that in this system, the copolymer exhibits the properties of an effective catalyst. The technical result is an increase in the specific energy, specific power and number of charge and discharge cycles of a metal-air current source.

This application is a continuation of U.S. Ser. No. 16/066,501, filedJun. 27, 2018, which is a nationalization of International applicationNo. PCT/IB2016/001998, filed Dec. 20, 2016 which claims priority to andbenefit of Russian Application No. 2015156759, filed Dec. 28, 2015, andthe entirety of these applications are incorporated herein by referenceherein.

FIELD OF TECHNOLOGY

The invention relates to electro-technical current sources, particularlylithium-air current sources and electrodes for them, and may be used tocreate various energy storage devices, for example, batteries with highspecific electrical characteristics.

PRIOR ART

Metal-air current sources usually include an anode made from an activemetal, and a cathode that is air-permeable—or more precisely—permeablefor molecular oxygen, separated by an electrolyte containing ions of themetal from which the anode is made. The cathode appears as a porous,electrically conductive structure with a highly developed surface,generally made of carbon material, on whose surface occurelectrochemical reactions that reconstitute and discharge molecularoxygen and discharge it from the oxygen-containing metal compound, forexample from the oxide or peroxide of the metal in the process of thedischarging and charging of the current source.

In particular, when lithium is used as the metal of the anode inso-called lithium-air current sources, the electrochemical processesthat occur are described in the following way.

When the lithium-air current source is discharged, oxidation of thelithium takes place on the anode, from which the lithium ions pass intothe electrolyte, while an electrochemical reconstitution of themolecular oxygen that enters from the surrounding atmosphere through theporous cathode to the cathode-electrolyte boundary, takes place on thecathode. The electrochemical reactions that occur in such a systemduring discharge are described as follow:

on the anode: Li-e=Li⁺,

on the cathode: O₂+4Li⁺+4e=2Li⁺+O₂+2e=Li₂O₂.

During the charging of such a current source, the oxygen contained inthe oxide or peroxide of lithium, is oxidized on the cathode intomolecular oxygen and returns back into the atmosphere. The lithium ionsare reconstituted into metallic lithium on the anode. Theelectrochemical reactions that occur in such a system during chargingare described as follow:

On the cathode: 2Li₂O-4e=4Li⁺+O₂ or Li₂O-2e=2Li⁺O₂

On the anode: Li⁺ +e=Li.

Lithium-air current sources have unique characteristics, in that thecathode-active material—oxygen—is not stored in the source, but comes infrom the surrounding atmosphere. A lithium-air current source has anopen-circuit voltage (EMF) in the order of 2.91 V, while its theoreticalaccounting specific energy is 11,140 W×h/kg [K. M. Abraham. “A BriefHistory of Non-aqueous Metal-Air Batteries”//ECS Transactions, 3 (42)67-71 (2008)]. Such current sources may be used, for example, asbatteries for automobiles, which require rechargeable current sourcesthat have a lifetime of at least 1000 charge-discharge cycles and aspecific power value of at least 400 W/kg.

Various metal-air current sources are known. Thus, in U.S. Pat. No.5,510,209, there is described a metal-air current source comprising ametallic anode, a composite carbon cathode and an electrolyte with highion conductivity, located as a polymer film between the anode and thecathode, on which there occur the processes of reconstituting molecularoxygen during discharge. As a metal for the anode, metals such aslithium, magnesium, sodium, calcium, aluminum and zinc are proposed foruse. This current source has a sufficiently high specific energyvalue—in the order of 3,500 W×h/kg (relative to the mass of thecathode); however, it has a low discharge current density, ranging from0.1 mA/cm² to 0.25 mA/cm²; in other words, it has a very low specificpower.

The particularities that are indicated are due to the low velocity ofthe electrochemical reactions that take place on the cathode, because ofthe high activation energy of these processes. Accordingly, asignificant number of the known inventions from prior art are connectedwith various improvements of the cathode, which, in the required manner,would affect the electrochemical properties of such current sources.

In particular, in order to increase the velocity of the reactionsindicated and to thereby increase the specific power of metal-aircurrent sources on the surface of the cathode, where the reconstitutionof molecular oxygen directly takes place and it is separated from theoxygen-containing metal compound, for example, from the oxide orperoxide of a metal, in the discharge-charge process of the currentsource, in one way or another, a catalyst is applied.

Thus, there is known a cathode for a lithium-air current source,described in application KR 20140056544 comprised of manganese dioxidewith the addition of nanoparticles of noble metals (platinum, palladium,ruthenium, iridium and gold), applied to a nickel grid. However, the useof precious metals in the cathode material results in the making theelectrode significantly costlier and also the current source in which itis used.

U.S. Pat. No. 7,087,341, there describes a metal-air current source,comprising an anode and a cathode, in which the cathode includes a gasdiffusion layer, a current collector and a layer with a catalystcomprising carbon particles, the average size of which does not exceed10 microns and particles of a catalyst. As catalysts, manganese oxide,cobalt oxide and nickel oxide are suggested. When a laboratoryelectrochemical cell was tested, which modeled such a current source,particularly with a mixture of nickel Ni(II) oxide and cobalt Co (II)oxide as a catalyst enabling the reconstitution of the oxygen of thecatalyst, the following values were obtained: specific power—35 W/kg,specific energy—80 W×h/kg. The number of charge-discharge cycles did notexceed 30. Clearly, such a catalyst does not provide the desired highoperating performance for a current source.

There is known a lithium-air current source, described in patent CN102240574, comprised of a lithium anode, a carbon cathode containing thecatalysts for the oxygen reaction, a separator and an organicelectrolyte. As a catalyst on the cathode, there are used complexes ofcobalt and manganese with pyridine, 4,4′-bipyridine, pyrazine andpyrrole. Monomer complexes that are used as catalysts are mixed with acarbon material in the process of manufacturing the cathode and areadsorbed on it. However, in the process of preserving and operating thecurrent source, the molecules of the catalyst that are weekly bondedwith the carbon material can dissolve in the electrolyte; as a result,the efficiency of the catalyst will noticeably decrease from onecharge-discharge cycle to the next.

Also known is the use of conductive polymers in metal-air currentsources. Thus, in application WO 2011/097286, there is described ametal-air current source whose cathode includes a gas diffusion layer,comprised of finely dispersed carbon, coated with the help of anelectrochemical or chemical method for applying the layer of conductivepolymer, for example, polythiophene and/or polypyrrole. It has beenshown that a conductive polymer somewhat improves the efficiency of theelectrochemical reconstitution of oxygen, as compared with finelydispersed carbon; however, it is insufficient for practical use.

Therefore, in addition, as a catalyst, it is proposed to use particlesof metals, for example, silver and/or oxides of metals. The role of theconductive polymer is essentially to physically hold back particles ofthe catalyst. This approach of physically bonding particles of thecatalyst with the help of conductive polymers is also used in otherchemical current sources, for example, in fuel cells, as shown inapplication CN 1674330A.

As can be seen from prior art, at the present time, metal-air currentsources, particularly the best of them, lithium-air current sources,have a lifetime in the order of several dozen charge-discharge cycles;at the optimum specific power, no more than several dozen W/kg. Here itshould be admitted that the electrical parameters of such currentsources are significantly dependent on the electrochemical properties ofthe cathode, particularly the efficiency of the catalyst for the oxygenreaction. The problem that this invention aims to solve is to create acathode for a metal-air current source possessing high electrochemicalactivity with respect to oxygen reactions, i.e. reactions for thereconstitution of molecular oxygen and reactions to separate oxygen,which, in turn, will make it possible to create a metal-air currentsource with improved characteristics in terms of specific energy,specific power and the number of charge-discharge cycles.

DISCLOSURE OF THE INVENTION

Application is being made for a group of inventions: a cathode and ametal-air current source, in which the above-mentioned cathode is used,forming a single inventive concept, i.e. to achieve the possible ofcreating metal-air current sources with improved characteristics interms of specific energy, specific power and the number ofcharge-discharge cycles.

One object of the invention is a cathode for metal-air current sources,including a base made of porous electrically conductive material that ispermeable to molecular oxygen, on the working surface of which there isapplied a copolymer obtained by means of the copolymerization of themonomer complex compound of a transition metal with A Schiff base and amonomer from the thiophene group.

Said copolymer consists of fragments of the above-mentioned complexcompound of a transition metal with A Schiff base and fragments of amonomer from the group of thiophenes that are present in the compositionof the copolymer in molar ratios, connected with the composition of themixture of the initial monomers (before polymerization). Each of theconstituent parts of the copolymer fulfils its function in theelectrochemical processes that occur on the cathode.

Fragments of the complex compound of transition metal with A Schiffbase—as has been discovered by the authors of the invention—turn out tobe highly effective reaction centers, i.e. a catalyst that is capable ofconcentrating the molecular oxygen that comes in through the porous baseof the cathode, and the metal ions coming from the electrolyte. Thisresults in a decrease in energy losses in the reaction to reconstitutethe oxygen and in an increase of its velocity, which provides anincrease in the specific energy and specific power of the current sourceas an energy storage system.

One of the reasons that limit the useful lifetime (number ofcharge-discharge cycles) of metal-air current sources is the blockage ofthe surface of the catalyst that is applied on the cathode by largenon-conductive and insoluble crystals of the oxide or peroxide of theactive metal. To the extent that fragments of the complex compound ofthe transition metal with a Schiff base that appear in such a system asa catalyst consist of individual reaction cents, the products of theelectrical reconstitution of the oxygen (oxide or peroxide of the activemetal) that are formed on them have a nanocrystal structure. Such astructure of the products of the electrical reconstitution oxygenprovides for their more complete oxidation when the current source ischarged. This makes it possible to obtain a greater number ofcharge-discharge cycles in the system.

The fragments of the monomer from the thiophene group that enter intothe composition of the copolymer promote an increase in the chargetransport velocity (or the electrical conductivity of the copolymer),which increases the velocity of the cathode reactions and raises thespecific power of the current source in which this cathode is used.Besides, the high electrical conductivity of the copolymer makes itpossible to increase its thickness while preserving catalytic activityon the entire layer of the copolymer, which together also provides ahigh specific energy for the current source.

From prior art there is known the use of conductive polythiophenes forthe manufacture of a current source cathode. Particularly,polythiophenes are used as a material for carbonization in order toobtain the carbon base for a cathode of a lithium-air battery (CN104518225); as a material that connects the current lead and thecatalyst of a fuel cell cathode (GB 201009720);

as a barrier layer for a cathode, protecting the components of alithium-air battery from moisture (US 20150079485); as a protectivelayer, protecting the active component of the cathode of a lithium-airbattery from the loss of oxygen from a crystal lattice (WO 2015149211).However, from prior art there is known the use of polymer materials on abase of complexes of transition metals with a Schiff base, both in theform of individual polymers and in in a composition of copolymers,including copolymers with thiophenes, as catalysts for the oxygenreaction in metal-air current sources.

As a material for the base of the cathode, it is preferable to use aporous carbon material with a developed surface. Carbon materials have alow density (specific weight), sufficient mechanical strength, a highdegree of surface development, which can be easily varied by knownmethods and, at the same time, they are chemically inert; they possessgood adhesion to the copolymer, which is proposed for use in accordancewith the present invention.

To obtain a copolymer as monomer complex compound of a transition metalwith a Schiff base, a compound of the form [M(R, R'Salen)] may be usedthat has the structure

wherein M is the transition metal, selected from the group nickel,palladium, platinum, cobalt, copper, manganese;

Salen is the residue of bis(salicylaldehyde)ethylenediamine in theSchiff base;

R is the substituent in the Schiff base, selected from the group H,CHO—, C₂H₅O—, HO or —CH₃;

R′ is the substituent in the Schiff base, selected from the group H orany of the halogens.

Also, to obtain the copolymer, as a monomer complex compound of atransition metal with a Schiff base, a compound of the form [M(R,R'Saltmen)] may be used that has the structure

wherein M is the transition metal, selected from the group nickel,palladium, platinum, cobalt, copper, manganese;

Saltmen is the residue of bis(salicylaldehyde)tetramethylethylenediaminein the Schiff base;

R is the substituent in the Schiff base, selected from the group H,CHO—, C₂HSO—, HO or —CH₃;

R′ is the substituent in the Schiff base, selected from the group H orany of the halogens.

Also, to obtain the copolymer as a monomer complex compound of atransition metal with a Schiff base, a compound of the form [M(R,R'Salphen)] may be used that has the structure

wherein M is the transition metal, selected from the group nickel,palladium, platinum, cobalt, copper, manganese;

Salphen is the residue of bis(salicylaldehyde)-o-phenylenediamine in theSchiff base;

R is the substituent in the Schiff base, selected from the group H,CHO—, C₂HSO—, HO or —CH₃;

R′ is the substituent in the Schiff base, selected from the group H orany of the halogens.

As a second component to obtain the copolymer, a monomer selected fromthe group: thiophene, 3-alkylthiophen, 3,4-dialkylthiophene,3,4-ethylenedioxythiophene (

or EDOT) or a combination thereof.

A monomer complex compound of a transition metal with a Schiff base anda monomer from the thiophene group, used to obtain the above-mentionedcopolymer, can be taken in a molar ratio ranging from 1:05 toapproximately 1:10, preferably 1:2, for example.

Another object of the invention is metal-air current source comprising acathode, as it is characterized above, including in the particular casesindicated when it is designed, and an anode made from a material thatincludes at least one chemically active metal, where the anode and thecathode are divided by an electrolyte that contains ions of thechemically active metal mentioned above that enters into the compositionof the anode.

As a material from which the anode of the metal-air current source ismade, use can be made of an alkali metal, an alkaline earth metal or atransition metal. Such metals have a negative electrode potential;therefore, they are preferably used as the material of the anode.

In particular, as an alkali metal, lithium can be used, which has themost negative electrode potential. In this case, as an electrolyte insuch a current source with a lithium anode, use may be made, forexample, of a solution of lithium trifluoromethanesulfonate in adimethyl ether of tetraethylene glycol with the molar ratio of thesecomponents ranging from 1:2 to approximately 1:8, preferably 1:4. Theindicated range is determined by the solubility of the salt of lithiumtrifluoromethanesulfonate in the solvent dimethyl ether of tetraethyleneglycol. The selection of the electrolyte is determined by the fact thatit provides high ion conductivity; it is stable in a wide range ofvoltages (area of electrochemical stability); moreover, lithium does notreact with it, which rules out a self-discharge of the lithium-aircurrent source with such an electrolyte.

Also as a material, from which the anode is made, an alloy may be use,including one or more chemically active metals. In particular, use maybe made of a lithium-silicon alloy, a lithium-aluminum alloy, alithium-tin alloy or a lead-sodium alloy. The above-mentioned alloyshave sufficient negative electrode potential and, at the same time, theyprovide for a higher thermodynamic (corrosion) stability and mechanicalstability of the anode.

BRIEF DESCRIPTION OF THE INVENTION

On FIG. 1 and FIG. 2 , as an example of an embodiment of the presentinvention, there is diagrammatically shown the construction of alithium-air current source, comprising a lithium anode and a cathode inthe form of a carbon base with a copolymer applied on it in accordancewith the present invention, illustrating the discharge process of such acurrent source. At the same time, FIG. 1 shows the state of the currentsource at the beginning of the discharge process, while FIG. 2 showsthis at the end of the discharge process.

FIG. 3 provides a diagrammatic representation of a fragment [Co(Schiff)]entering into the composition of the copolymer, while FIG. 3 representsits graphic formula, while FIG. 3(b) provides a diagrammaticrepresentation of the fragment [Co(Schiff)] corresponding to its spatialarrangement.

FIG. 4 provides a diagrammatic representation of the spatial stackstructure that is formed in the copolymer from the fragments[M(Schiff)], particularly [Co(Schiff)].

FIG. 5 diagrammatically shows the interaction of molecular oxygen withthe fragments [Co(Schiff)] of the copolymers according to the presentinvention.

FIG. 6 illustrates the interaction of the lithium ions with thefragments [Co(Schiff)] of the copolymer, while FIG. 6(a) represents thegraphic formula of a fragment [Co(Schiff)] of the copolymer interactingwith the lithium ions, while FIG. 6(b) illustrates such an interactionwith the corresponding spatial arrangement of the lithium ions and theindicated fragment of the copolymer interacting among themselves.

FIG. 7 represents the results of the experiment to determine thecorrelation of the concentrations of the monomer complex compound of thetransition metal with a Schiff base and a monomer from the group ofthiophenes used to obtain the copolymer according to the presentinvention.

FIG. 8 represents the curves of the charging and the discharge of anelectrode with a copolymer coating according to the invention and acontrol electrode.

FIG. 9 represents the curves of the charging and the discharge of alithium-air current sources with a cathode according to the inventionand a control example of a current source.

EMBODIMENT OF THE INVENTION

A possibility for an embodiment of the present invention is shown belowin the example of a lithium-air current source (cf. FIG. 1 and FIG. 2 ),comprising lithium anode 1 and cathode, comprising base 3 made of aporous electrically conductive material that is permeable to molecularoxygen with a coating made of copolymer 4 applied to it, obtained by thecopolymerization of a monomer complex compound of cobalt with a Schiffform base [Co(Schiff)] and a monomer from the thiophene group, in thiscase 2,3-ethylendioxythiophene (EDOT). Mechanically, anode 1 and cathode2 are divided by separator 5, but electrochemically by electrolyte 6containing lithium ions 7. Copolymer 4 may be applied on the surface ofbase 3 of cathode 2 by electrochemical polymerization from a solution ofa mixture of monomer [Co(Schiff)] and monomer EDOT. As a material forbase 3, use may be made of a material containing carbon of the brandCarbon Super • produced by the company TIMCAL. As shown on FIG. 1 andFIG. 2 , in the structure of copolymer 4, fragments of complex[Co(Schiff)], including metallic center 8 and ligand environment(ligand) 9 and EDOT 10 fragments can be separated.

Studies that have been conducted, including by one of the authors of thepresent invention, have shown that the polymer complexes of the compoundof a transition metal with a Schiff base have a specific stack structurewith fragments of the polymer connected with one another by means ofdonor-acceptor interaction between the metallic center of one fragmentof the polymer and the ligand of another fragment of the polymer [I. E.Popeko, V. V. Vasiliev, A. M. Timonov, G. A. Shagisultanova.“Electrochemical Behavior of Palladium (III) with Schiff's Bases,Synthesis of Mixed-Valent Pd(II)-Pd(IV) Complexes”//Russian J. Inorg.Chem. 1990, V. 35, N. 4, p. 933]. FIG. 3 provides a diagrammaticrepresentation of a fragment [Co(Schiff)] entering into the compositionof copolymer 4, including metallic center 8 and ligand environment(ligand) 9. In this example, metallic center 8 is cobalt (Co), whileligand 9 is Salen. FIG. 4 gives a diagrammatic representation of thespatial stack structure that is formed in copolymer 4 from the fragments[Co(Salen)] and in which these fragments are arranged in parallel,following one another, so that, in order for metallic center 8 to bearranged immediately above and below ligands 9 of the adjacentfragments, which are essential for previously mentioned alignment of thestack structure, thanks to the donor-acceptor interaction.

The possibility for achieving the indicated result, with respect to theenergy parameters of the current source under consideration is connectedto the properties of the above-mentioned copolymer discovered by theauthors of the present invention.

Fragments of [Co(Schiff)] form have a strong chemical affinity tomolecular oxygen; in an air environment, such structures are capable ofinteracting with molecular oxygen by forming bridges of a“metal-oxygen-metal” type between metallic centers [EI-Ichiro Ochiai.“Electronic structure and oxygenation ofbis(salicylaldehyde)ethylenediminicobalt(II)”//J. Inorg. Nucl. Chem.1973, V. 35, p. 1727]. FIG. 5 shows such an interaction of molecularoxygen with metallic centers 8 of fragments [Co(Schiff)] of copolymer 4.In particular, for the polymer poly-[Co(Schiff)], it has been shown thatthe concentration of oxygen in it is approximately 500 times higher thanin air, while such oxygen 12 connected with the polymer has a bond thatis longer—and thereby weakened—between the oxygen atoms than a moleculeof free molecular oxygen. This means that the bonded oxygen hastransitioned to more active state because of the action of the fragment[Co(Schiff)] that has presented itself as a catalyst in such a system.

Also fragments of a monomer from the thiophene group entering into thecomposition of the copolymer promote an increase in the electricalconductivity of the copolymer, which increases the velocity of thecathode reactions and raises the specific power of the current source inwhich this cathode is used. Besides this, the high electricalconductivity of the copolymer makes it possible to increase itsthickness while preserving the catalytic activity in the whole layer ofthe copolymer, which, taken together, also provides for the highspecific energy of the current source.

In studying cathodes of a lithium-air current source in relation to thepresent invention, it has been established that the use—in thecomposition of a cathode—of a coating made of a copolymer obtained bycopolymerization of a monomer complex compound [M(Schiff)] and a monomerfrom the thiophene group, other things being equal, leads to an increasein the discharge current of the cathode as compared to an analogouselectrode, in which a coating made of a polymer poly-[M(Schiff)] isused.

To determine the correlation of the monomer complex compound [M(Schiff)]and the monomer from the thiophene group in the mixture that is used toobtain the copolymer, wherein the indicated result of the invention isachieved, including an optimal value for the indicated correlation, thefollowing experiment was done, including the manufacture and testing ofelectrodes with a differing polymer coating with respect to itscomposition.

To a glass-graphite electrode (surface area 0.07 cm²) there was applieda coating of acetonitrile solvents containing a monomer complex compound[M(Schiff)], in particular a complex [Co(CH₃O-Salen)] and a compoundfrom the thiophene group—EDOT.

Here, the value of the concentration of the compound [Co(CH₃O-Salen)]was set as constant and equal to 1 mmol/L, while the value of theconcentration of EDOT for different examples of electrodes was variedfrom zero to 10 mmol/L. In addition, the solution included thebackground electrolyte LiBF₄ in a concentration of 0.1 mol/L. Theapplication of the copolymer coating was carried using the method ofelectrochemical polymerization with cyclical variation of the potentialof the electrode within a range from 0 V to +1.5 V (in relation to asilver-silver chloride electrode) with a velocity of 50 mV/sec (2 cycleswere carried out). After this, the electrode was washed in acetonitrileand was dried for 2 minutes at room temperature.

An electrode obtained in this manner was placed into a three-electrodeairtight electrochemical cell, filled with 0.1 mol/L of LiBF₄ inacetonitrile. As an auxiliary electrode a glass-graphite plate measuring1.5×1.0 cm was used; the comparison electrode was an Ag⁺/Ag electrodefilled with 5×10⁻³ mol/L of a solution of AgNO₃ in acetonitrile (astandard electrode of the brand MF-2062 produced by the companyBioanalytical Systems, BASi). The electrode studied was subjected to acharge in voltammeter mode while shifting the potential from 0 V to +1.3V with respect to the silver chloride electrode with a velocity of 50mV/sec. After that, the electrode was subjected to a discharge involtammeter mode while shifting the potential from +1.3 V to −0.7 V withrespect to the silver chloride electrode at a velocity of 50 mV/sec,fixing the discharge current at a potential value of 0.3 V.

The results of the experiments are shown in FIG. 7 , where, on the xaxis, there are shown the values of the K correlations of the molarconcentrations of EDOT and the [Co(CH₃O-Salen)] compound, while, on they axis, there are the values of the discharge current I that wereindicated, normalized to the discharge current of the electrode with thecoating that had been obtained at zero concentration of EDOT−I(EDOT=0).The experimental values and the approximating curve are shown. It can beseen that even a small addition of EDOT to [Co(CH₃O-Salen)] in themixture that is used to obtain the copolymer leads to an increase in thedischarge current of the electrode. Apparently, this is related to theincrease in the electron conductivity of the copolymer and animprovement in charge transport conditions in it. Furthermore, theaddition of EDOT fragments that are good conductors of a charge makes alarger number of the fragments [Co(Schiff)] available for theelectrochemical reaction on the electrode. In other words, for allpractical purposes, it increases the number of active metallic centersin the copolymer. From this standpoint, the greatest efficiency is shownby the copolymer that is obtained from the solution with a concentrationratio of [Co(CH₃O-Salen)] and EDOT of 1:2 (the maximum on the curveshown in FIG. 7 corresponds to this).

It can be seen that, in range from approximately 1:0.5 to approximately1:10 for the values of the relationship of the concentrations of[Co(CH₃O-Salen)] and EDOT, there is observed a high operationalefficiency of the electrode. When the EDOT content is further increased,there is observed a noticeable decrease in the discharge currents in theelectrode under consideration, which can probably be related to thephysical blockage of the catalytic cobalt centers by fragments ofthiophene.

We will consider the charge and discharge processes of a current sourcein relation to the present invention.

Process of Discharging

In the process of discharge of a lithium-air current source (cf. FIG. 1), atmospheric oxygen 11 penetrates through base 3 of cathode 2 andconnects with fragments of the [Co(Schiff)] complex of copolymer 4,passing over into a more active state (position 12 on FIG. 1 ). Lithiumanode 1 is oxidized with the formation of lithium ions 7, which begin tomove in the direction of cathode 2. Furthermore, lithium ions 7 areattracted to fragments of the [Co(Schiff)] complex of copolymer 4 ofcathode 2 by the oxygen atoms of ligand 9, as illustrated in FIG. 6 .Here, FIG. 6(a) presents the graphic formula of the fragment of the[Co(Schiff)] complex of copolymer 4, interacting with the lithium ions,while FIG. 6(b) provides an illustration of such an interaction, inwhich lithium ions 7 are attracted to the negatively charged oxygenatoms of ligand 9 of the [Co(Schiff)] fragment of copolymer 4. Theexcess of electrons in copolymer 4 leads to the reconstitution of boundoxygen 12. The products of the reconstitution are stabilized by lithiumions 7 in the form of a nanocrystal oxide or peroxide of lithium 13 (cf.FIG. 2 ).

The described reaction that reconstitutes the oxygen proceeds veryrapidly, inasmuch as the reconstituted oxygen and the lithium ions areconcentrated in one and the same reaction zone of the [Co(Schiff)]fragment of the copolymer at a close distance from one another, whichfacilitates the chemical interaction between the lithium and the oxygen,leading to the formation of an oxide or peroxide of lithium. Thecatalysts used for the reconstitution, as a rule, adsorb and concentrateonly one reagent, usually the oxygen. The [Co(Schiff)] fragments of thecopolymer that exhibit catalytic properties “attract” both the lithiumions and the oxygen. The process of discharge ends after whole surfaceof the cathode is coated with a thin coat of the products of thedischarge.

Process of Charge

In the process of charging a current source, designed in accordance withthe present invention, as a result of applying a positive electricalcharge to cathode 2 with respect to anode 1, metallic centers 8 of the[Co(Schiff)] fragments of polymer 4 are oxidized and pass over into anoxidized state with a degree of oxidation of +3.

The metallic centers—in this case, cobalt atoms—in such an oxidizedcondition are powerful oxidizing agents capable of oxidizing an oxide orperoxide of lithium back into molecular oxygen and lithium ions. Themolecular oxygen leaves the reaction zone and escapes into thesurrounding atmosphere through the porous carbon material of base 3 ofcathode 2, while lithium ions 7 diffuse back toward lithium anode 1,where they are reconstituted into metallic lithium. Copolymer 4, in thiscase, acts as an electrochemical catalyst, remaining in an oxidizedstate, thanks to the positive potential that has been applied to cathode2 from an outside power source.

In the process of discharge that has been described, the coating of thecathode remains stable in the whole range of the operational potentials;no irreversible changes in the structure of the copolymer take place. Asa result of the charging of the lithium-air current source underconsideration, the oxide (peroxide) of lithium actually turns back intooxygen and lithium ions, while the surface of the cathode is freed fromthese products that were formed in the process of the discharge of thecurrent source. All of this together makes it possible to substantiallyincrease the number of charge-discharge of the current source ascompared to those known.

Example 1. Charge-Discharge Process of an Electrode with a CopolymerCoating

Production of an electrode. As a base for the electrode, there wasselected a glass-graphite disk with a diameter of 22 mm (surface area0.07 cm²) produced by the company BASi (MF 2012). To the working surfaceof the electrode, using the method of electrochemical polymerization,there was applied copolymer from an acetonitrile solution containing1×10³ mol/L of a monomer of complex compound of cobalt with Schiff base[Co(CH₃O-Salen)], 2×10³ mol/L of EDOT and 0.1 mol/L of backgroundelectrolyte tetraflouroborate tetraethylammonium (C₂H₅)₄NBF₄. Thepolymerization was carried out in an airtight case filled with argonwith a total concentration of water and oxygen of less than 10%. Theprocess comprised two cycles of changing the potential of the electrodein a range from 0 V to +1.5 V with respect to the silver-silver chlorideelectrode with a velocity of 400 mV/sec. After the end of the process ofpolymerization, the electrode was washed with acetone nitrile andsubjected to drying for 2 minutes at room temperature.

Testing of the electrode. The electrode was placed in a three-electrodeairtight electrochemical cell, filled with 0.1 mol/L of a solution ofLiBF₄ in acetone nitrile saturated with oxygen by preliminary blowing ofsaid solution for 15 min.

As an auxiliary electrode, a glass-graphite rectangle plate measuring1.5×10 cm was used; the comparison electrode was an Ag+/Ag electrode,filled with 5×10.3 mol/L of a AgNO₃ solution in acetone nitrile (astandard electrode of the brand MF-2062 produced by the company BASi).

The electrode was subjected to discharge and subsequent charging at aconstant current of 13 μA. Analogous testing was conducted on similarsuch electrode without the layer of copolymer applied to its surface,i.e. a control electrode. The charge-discharge curves of the describedelectrodes are shown in FIG. 8 , where the x axis shows thecharge/discharge time, while the y axis shows the potential value on theelectrode. It can be seen that the discharge capacity (which isreflected by the discharge time) of the electrode coated with thepolymer significantly exceeds the potential value for the controlelectrode. Here, the potential of the electrode with the copolymer atthe time of discharge is significantly higher, but at the time ofcharging—it is significantly lower than corresponding potential valuesfor the control electrode. This reflects the fact that the copolymerexhibits catalytic activity with respect to the processes of thecharging and the discharge of the electrode. In turn, this provides forimproved characteristics of the specific energy and specific power ofthe current source, in which this electrode will be used.

For convenience, in the experiment, as the base of the electrode, therewas used glass-graphite, which is not a porous material that ispermeable for oxygen. However, inasmuch as the processes of charging anddischarge occurring in the polymer were studied, while the supply ofoxygen into the reaction zone was carried out by means of priorsaturation of the electrolyte with oxygen, the results of the studiesadequately reflect the process occurring on the cathode in accordancewith the present invention.

Example 2. Charging and Discharge of a Lithium-Air Current Source

Production of the electrodes and current source. In the production ofthe cathode, carbon material (of the brand Super P produced by thecompany TIMCAL) 80% by mass and a binding agent—polyvinylidene fluoride(of the brand HSV 900 produced by the company Arkema)—20% by mass weremixed in a solvent N-methyl-2-pyrrolidone (produced by the companySigma-Aldrich). The mass obtained was uniformly applied on to Toray-30gas-permeable carbon paper (Toray Carbon Paper TGP-H-030) and thepreparation was subjected to drying for 12 hours at a temperature 120°C. to remove the residues of the solvent. The density of the applicationof carbon on the obtained base of the electrode was (0.9±0.1) mg/cm².Then, onto the base of the cathode that was obtained in this way, therewas applied a coating made of a copolymer.

The process of application was carried in an airtight case filed withargon and with a total concentration of water and oxygen of less than10⁻⁵%. The process of polymerization was carried out in an acetonenitrile solution containing 1.0 mmol/L of the monomer [Co(CH₃O-Salen)]and 2.0 mmol/L of the monomer EDOT, and also a background electrolyte−0.1 mmol/L of C₂H₅)₄NBF₄, at a potential of +1.5 V with respect to thesilver-silver chloride electrode for 2 sec.

The anode was made of lithium foil with a thickness of 500 microns. Thecurrent source was collected in a steel case, type R2032 (coin-type). Inthe lid of the case, in contact with the cathode, which was a currentcontact jaw, there 21 openings with a diameter of 1 mm to provide forthe access of oxygen to the cathode. The cathode and the anode weredivided by a porous separator Celgard 2500 (produced by the companyCelgard, LLC) with a thickness of 25 mm. As a lithium-containingelectrolyte, 1 mol/L of a solution of lithium trifluoromethanesulfateLiFCF₃SO₃ (produced by the company Aldrich) was used in tetraethyleneglycol dimethyl ether (TEGDME) produced by the company Acros.

Also, a control current source was prepared, differing from the onedescribed—the experimental one—filled according to the presentinvention, only in that its cathode did not have the indicated copolymercoating.

Both current sources (experimental and control) were tested underidentical charge-discharge conditions on a CT-3008W unit produced by thecompany NEWARE (KHP). The charge was produced at a constant current of50 μA and the discharge at a constant current of 500 μA. In the processof testing, both current sources were in an oxygen atmosphere (at apressure of 1 atm) at room temperature.

FIG. 9 shows the experimental charging and discharge curves that wereobtained for the control and the experimental current source. On the xaxis, there is shown the specific capacity value (calculated on 1 gcarbon), while on the y axis—the voltage value U of the current source.It can be seen that, in a lithium-air current source, the use of acathode with copolymer coating according to the present inventionprovides a greater discharge voltage, and consequently, greater energyrelease upon discharge, but also less voltage, essential for thecharging.

Despite the fact that, in the examples, examples are cited that wereobtained while using, in the current source, complexes of cobalt with aSchiff base and EDOT, analogous results are shown with the use of othercopolymers obtained from a mixture of metal complexes with a Schiffbase, for examples, complexes of nickel, manganese and other transitionmetals, and monomers of the group of thiophenes.

Thus, the results of the experiments confirm that the use, in metal-aircurrent sources, of a cathode, whose working surface has a coating madeof a copolymer obtained by copolymerization of a monomer complex of acompound of a transition metal with a Schiff base and a monomer from thethiophene group leads to high energy characteristics for such currentsources as compared to analogous current sources that do not contain theindicated polymer in the composition of the cathode. This is achievedthanks to the fact that the polymers indicated, as was discovered by theinventors, in such a system, act as catalysts of cathode reactions.Here, the high electrical conductivity of the copolymer makes itpossible to increase its thickness while preserving the catalyticactivity in the entire layer of the copolymer, which, taken together,provides for both the high specific power and the high specific energyof the current source. The reversibility of the reactions of oxidationand reconstitution on the cathode provides for a long useful life of thecurrent source.

The invention claimed is:
 1. Cathode for metal-air current sources,comprising a base made of a porous electrically conductive material thatis permeable for molecular oxygen, on whose working surface there isapplied a copolymer, obtained by polymerization of a monomer complex ofa complex compound of a transition metal with a Schiff base and amonomer from the thiophene group.
 2. Cathode according to claim 1, inwhich, as the material for the base, a porous carbon material with adeveloped surface is used.
 3. Cathode according to claim 1, in which, inorder to obtain the copolymer, as a monomer of a complex compound of atransition metal with a Schiff base, a compound of the form [M(R,R'Salen)] is used, having the structure

wherein M is the transition metal, selected from the group nickel,palladium, platinum, cobalt, copper, manganese; Salen is the residue ofbis(salicylaldehyde)ethylenediamine in the Schiff base; R is thesubstituent in the Schiff base, selected from the group H, CHO—, C₂H₅O—,HO— or —CH₃; R′ is the substituent in the Schiff base, selected from thegroup H or any of the halogens.
 4. Cathode according to claim 1, inwhich, in order to obtain the copolymer, as a monomer complex compoundof a transition metal with a Schiff base, a compound of the form [M(R,R'Saltmen)] may be used that has the structure

wherein M is the transition metal, selected from the group nickel,palladium, platinum, cobalt, copper, manganese; Saltmen is the residueof bis(salicylaldehyde)tetramethylethylenediamine in the Schiff base; Ris the substituent in the Schiff base, selected from the group H, CHO—,C₂H₅O—, HO— or —CH₃; R′ is the substituent in the Schiff base, selectedfrom the group H or any of the halogens.
 5. Cathode according to claim1, in which, in order to obtain the copolymer, as a monomer complexcompound of a transition metal with a Schiff base, a compound of theform [M(R, R'Salphen)] is used that has the structure

wherein M is the transition metal, selected from the group nickel,palladium, platinum, cobalt, copper, manganese; Salphen is the residueof bis(salicylaldehyde)-o-phenylenediamine in the Schiff base; R is thesubstituent in the Schiff base, selected from the group H, CHO—, C₂H₅O—,HO or —CH₃; R′ is the substituent in the Schiff base, selected from thegroup H or any of the halogens.
 6. Cathode according to claim 1, inwhich, in order to obtain the copolymer, as a monomer from the thiophenegroup, a compound, selected from the group thiophene, 3-alkylthiophen,3,4-dialkylthiophene, 3,4-ethylenedioxythiophene or a combinationthereof.
 7. Claim according to claim 1, in which the monomer complexcompound of a transition metal with a Schiff base and a monomer from thethiophene group used to obtain the indicated copolymer are taken in amolar ratio from approximately 1.05 to approximately 1:10.
 8. Cathodeaccording to claim 7, in which the monomer complex compound of atransition metal with a Schiff base and the monomer from the thiophenegroup are taken in a molar ratio of approximately 1:2.
 9. Metal-aircurrent source comprising a cathode, designed according to claim 1, andan anode, produced from a material, comprising at least one chemicallyactive metal, while the anode and the cathode are separated by anelectrolyte containing ions of the indicated chemically active metalthat enters into the composition of the anode.
 10. Current sourceaccording to claim 9, in which, as a material from which the anode ismade, an alkali metal, an alkali earth metal or a transition metal isused.
 11. Current source according to claim 10, in which, as the alkalimetal indicated, lithium is used.
 12. Current source according to claim11, in which, as an electrolyte, a solution of lithiumtrifluoromethanesulfonate in a dimethyl ether of tetraethylene glycol isused, with the molar ratio of these components ranging fromapproximately 1:2 to approximately 1:8.
 13. Current source according toclaim 12, in which the ratio of the indicated components of theelectrolyte is 1:4.
 14. Current source according to claim 9, in which,as material from which the anode is made, an alloy is used, comprisingone or more chemically active metals.
 15. Current source according toclaim 14, in which, as the indicated alloy, a lithium-silicon alloy,lithium-aluminum alloy, a lithium-tin alloy or a lead-sodium alloy isused.